More specifically, the present application relates to a method for operating an NMR probehead with an MAS stator for receiving a substantially circular-cylindrical hollow MAS rotor with an outer jacket of external diameter D and with a sample substance in a sample volume. The MAS rotor is mounted on pressurized gas in a measuring position within the MAS stator via a gas supply device and is set into rotation about the cylinder axis of the MAS rotor by a pneumatic drive, at a rotation frequency f≥30 kHz. During the NMR-MAS measurement, a temperature control gas is blown by a temperature control nozzle onto the outer jacket of the MAS rotor at an angle α<90° with respect to the longitudinal axis of the cylinder-symmetrical MAS rotor.
Such a method and a corresponding NMR probehead with an MAS stator and an MAS rotor are known, for example, from U.S. Pat. No. 7,915,893 B2.
Nuclear magnetic resonance (NMR) spectroscopy is a commercially widespread method in MR for characterizing the chemical composition of substances. In MR, the measurement sample which is situated in a strong static magnetic field is generally irradiated by radiofrequency (RF) pulses, and the electromagnetic reaction of the sample is measured.
Furthermore, it is known in solid-state NMR spectroscopy to rotate an NMR sample tilted at the so-called “magic angle” of approximately 54.74° in relation to the static magnetic field during the spectroscopic measurement (“MAS”=Magic Angle Spinning) in order to minimize line broadening on account of anisotropic interactions. To this end, the sample is inserted into an MAS rotor. MAS rotors are cylindrical tubes which are sealed with one or two caps, the upper one being provided with blade elements (“impeller”). The MAS rotor is arranged in an MAS stator, and the MAS rotor is driven to rotate by gas pressure by way of the blade elements. The totality of MAS rotor and MAS stator is referred to as MAS turbine.
The MAS turbine is arranged in an NMR-MAS probehead during the NMR measurement. The probehead comprises a cylindrical shielding tube. Housed therein are RF electronic components, in particular RF coils, and the MAS turbine. With its shielding tube, the probehead is typically inserted from below into the vertical room-temperature bore of a superconducting magnet, positioned therein and held therein with hooks, supports, screws or the like. The MAS turbine is then situated precisely in the magnetic center of the magnet.
During the rotation of samples, the rotor wall heats on account of the air friction. The central area exactly between the air bearings experiences the greatest heating. This leads to                1. non-uniform temperature distribution across the sample substance, and        2. undesired heating of the sample substance, such that a lower target temperature cannot be set or such that the sample is actually destroyed by heat.        
The temperature of the rotor is usually controlled via so-called VT channels or nozzles, which are directed approximately at the rotor center. Temperature-controlled gas is either injected into the radial bearings, if appropriate branched away from there (so-called VTN design), or delivered via a separate channel, wherein the bearings are then operated (so-called DVT design) at a VT-independent temperature, for example room temperature.
A predominant feature of the VTN and DVT solutions lies in the configuration of the temperature control nozzles. On account of the positioning and/or the size of the temperature control nozzles, they introduce the gas such that the gas speed directly at the rotor is considerably lower than the circumferential speed of the rotor, with the result that the cooling outlay is very high in order to control the temperature of the sample even just slightly below the room temperature (see the comparison table below).
A further disadvantage is that in all cases a temperature gradient establishes itself along the length of the rotor, which is undesirable; i.e. at the center of the rotor there is heating to ca. 55° C. at a total length of only 6 mm (see in the drawing the figure “Simulation report”).
This problem of temperature control is particularly pronounced in the case of small diameters D≤1.9 mm.
Since the temperature regulation and in particular the cooling of MAS rotors represent a very considerable aspect of solid-state spectroscopy, there are various strategies for improving these.
U.S. Pat. No. 5,289,130 has described the strategy of using the outflowing drive gas for cooling the rotor. Here, the gas is guided parallel to the rotor surface and guided off at the center. In this gas delivery, it is assumed that the gas speed on impacting the rotor is already too low to significantly cool the rotor. If the gas speed is still high enough, then this is associated with a loss of efficiency at the drive.
An arrangement of the type in question, except for the temperature control nozzles, has most of the features defined at the outset, and is already known from the products offered by Bruker BioSpin GmbH, as is shown at the Internet site
https://www.bruker.com/de/products/mr/nmr/probes/probes/solids/very-fast-mas/13-mm/overview.html
and from the publication by D. Wilhelm et al. “Fluid flow dynamics in MAS systems”, Journal of Magnetic Resonance 257 (2015) 51-63.
The document U.S. Pat. No. 7,915,893 B2, likewise cited at the outset, describes an arrangement which is entirely of the type in question and which has all of the features defined at the outset. In addition, a VT nozzle is shown which impinges obliquely on the rotor. The document describes a cryogenic probehead in which the coils are brought to cryogenic temperature, while the temperature of the measurement sample is controlled separately. Here, therefore, a spatial separation of electronics and rotor is necessary. The so-called spinner (rotor) is located in an outwardly closed pipe and its temperature is controlled via a temperature control nozzle. The temperature-controlled gas is distributed from a common pressure line to the bearing nozzles and the additional temperature control nozzles.
However, U.S. Pat. No. 7,915,893 B2 does not disclose how temperature control can be realized in an open system in which the rotor is not located in a narrow pipe. Nor does it disclose any relationship between the efficiency of the temperature control of the rotor and the velocity of the gas flow.
A considerable disadvantage of all of the temperature control devices known hitherto for MAS rotors is evident in operation at high speeds of the rotors during the measurement. Particularly at rotation frequencies f≥50 kHz, the air friction causes the rotors to become hot, which can lead to undesired changes or even to destruction of the measurement sample.